Estimation of the biological variation of glucose-6-phosphate dehydrogenase in dried blood spots

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Models based on biological variation provide a well-accepted database with reliable information for clinical laboratories for all purposes including screening, diagnosis and follow-up. Newborn screening laboratories use a blood spotted paper matrix to measure the analytes. This matrix medium is certainly different than body fluid matrix medium. After long-term monitoring of the performance of the Glucose-6-Phosphate Dehydrogenase Kit (R&D Diagnostics OSMMR 2000-D G6PD), the results obtained from the variation analysis were statistically evaluated. Analytical coefficient of variation, CVA, was found to be 5.41%. The CVA derived from between run assays was 5.32%, while within-subject biological coefficient of variation, CVI, was 7.26%. Since minimum performance is defined as CVA< 0.750 CVI, CVA should be lower than 5.44%. The analytical bias in calculation of total error \(B_{\rm A} < 0.375\break \sqrt {\left( {\it CV_{\rm I}} \right)^2 + \left( {\it CV_{\rm G}} \right)^2}\) was chosen to evaluate the performance of this assay. In this aspect, CVG, between-subject biological coefficient of variation, was found to be equal to 10.35%. BA was found to be 4.12%, which is lower than 4.74%, which means that it is acceptable. Therefore, the minimum quality specification for total error allowable is \({\it TE}_{\rm a} < 1.65(0.75\,CV_{\rm I} ) + {\rm }0.375\sqrt {\left( {\rm CV_{\rm I}} \right)^2 + \left( {\rm CV_{\rm G}} \right)^2 }\) . When the relevant results obtained in this study were substituted in this formula, TEa was found to be 13.7% for G6PD measurement in dried blood spots on paper filter matrix. It is expected that this figure will be helpful for the performance evaluation of newborn screening laboratories performing G6PD screening. We have been using error grid graphs for the evaluation of our external quality assurance survey results for the last two years, only because there was no data for assays employing filter matrix. Even the TEa already reported for EDTA whole blood samples used in G6PD assays has been remarkably high, which can easily create the wrong impression that G6PD is not a reliable test to perform from blood spot cards. The present study shows that this assay is adequately reliable even when performed from dried blood spot matrix. However, we believe that the combination of total allowable error, error grid graphs with a well-defined cut off is the best approach to obtain an accurate performance evaluation for this test.